Development of Aeromonas salmonicida subsp. masoucida vaccine in turbot and evaluation of protection efficacy under field conditions

Abstract Aeromonas salmonicida is a global fish pathogen that can infect a series of fish species. Vaccination by commercial vaccine has been proved to be an effective method to control this disease in Atlantic salmon. However, commercial vaccines used to protect against A. salmonicida subsp. salmonicida infection are not necessarily effective against infections by other subspecies. Given increasing number of infections by A. salmonicida subsp. masoucida in turbot (Scophthalmus maximus) farming in China, we developed an A. salmonicida subsp. masoucida inactivated vaccine for turbot in the present research. Using a previously isolated pathogenic strain RZ6S-1, a water-based vaccine and an oil-based Marcol 52 emulsified vaccine were developed, and first evaluated under laboratory conditions. Results showed that the relative percent of survival (RPS) and specific serum antibody titers of the fish vaccinated by the emulsified vaccine were significantly higher than that of those vaccinated by the water-based vaccine at 4 weeks post vaccination. Under field conditions, turbot were protected for 12 weeks and 24 weeks post vaccination by emulsified vaccine, with the challenged RPS reaching 84.66 ± 6.39% and 84.54 ± 5.93%, respectively. The specific antibody titers against A. salmonicida subsp. masoucida in the vaccinated fish could even be detected at 24 weeks post vaccination. In comparison, cumulative mortality of the control fish (12.96%) was higher than that of the vaccinated fish (2.48%), with more fish died of natural infection by A. salmonicida (221 vs. 17). Cutaneous nodules, possibly caused by chronic infection by A. salmonicida, were found in 23 of 30 randomly selected fish in the control pond at 24 weeks post vaccination, in contrast to only 1 of 30 in the vaccinated pond. More infections caused by Edwardsiella piscicida and scuticociliate were also observed in the control pond, indicating that the cutaneous nodules may facilitate the infection by other pathogens and parasites. Additionally, the fish weight in the vaccinated pond was significantly higher than that in the control pond, both at 12 weeks and 24 weeks post vaccination. These results indicated that the A. salmonicida emulsified vaccine can protect turbot for at least 24 weeks without severe side-effects on the fish growth under field conditions, and can also protect fish from chronic infection, which in turn reduces the infection probability by other pathogens or parasites. In conclusion, the present study indicates that the developed vaccine protects turbot from A. salmonicida subsp. masoucida infection under farming conditions.